The present invention relates to calculating the quality of vision and optical prescriptions for correcting optical imperfections in a patient's eyes.
Clinical optical prescriptions are calculated to determine the best combination of spherical and cylindrical lenses that optimize visual acuity for distant objects, thereby maximizing the quality of the retinal image. However, it has been found clinically that optimizing a prescription in the presence of high order aberrations as defined by many of the optic metrics used today does not always result in a prescription that causes the best focus as determined by a patient. Therefore, once an optic prescription for spherical and cylindrical correction is calculated, a clinician typically must use an iterative method for fine tuning that calculation in a trial and error fashion, as the patient compares the effect on vision while the clinician changes between slightly different corrective lenses. Since this method is time consuming, subjective, and requires clinical experience to practice accurately and efficiently, a method for objectively calculating an optimal clinical prescription without an iterative trial and error process is desired.
Further, in determining an optimal optical prescription, a clinician must also take into account the so-called refractionist's rule “maximum plus to best visual acuity.” This requires a clinician to calculate a prescription wherein the spherical component of a myopic (nearsighted) eye is slightly under-corrected and a hyperopic (farsighted) eye is slightly overcorrected. Thus, currently, the calculation for the ideal optical focus is not necessarily the best clinical prescription, requiring a second calculation or iterative testing to determine the optimal optic prescription. Because iterative calculations or testing is time consuming, a method is needed for incorporating the “maximum plus to best visual acuity” rule into a method for optimizing clinical optic prescriptions.
Furthermore, many of today's eyecare clinics employ commercial aberrometers to measure the eye's higher order aberrations, and the lower-order, sphero-cylindrical aberrations are readily measured with an autorefractor. Modern clinical aberrometers (e.g., Shack-Hartmann wavefront-sensing technique or subjective ray-tracing technique) typically provide detailed maps of the eye's refracting properties across the entire pupil. Such aberration maps have the potential to accurately form the basis of wavefront-guided corrections of the eye (e.g., refractive surgery, IOLs, and contact lens design. However, detailed aberration maps contain a wealth of information that can overwhelm a clinician and impede interpretation. One way to reduce the complexity of an aberration map is to fit the map with a relatively small number of Zernike radial polynomials, simplifying the description. However, even a Zernike spectrum is complex, and its relationship to visual quality is complicated. Ultimately it is desired to reduce an aberration measurement to a single metric of quality of vision, allowing a clinician to objectively evaluate the current quality of vision and the potential for improvement. Such an objective definition of the visual quality of the eye in a single metric would allow the evaluation of various methods of treatment (e.g., spectacle, IOL, contact lens, refractive surgery, etc.), and would be greatly appreciated in the art.
One strategy to simplify an aberration measurement into a metric of image quality includes the use of the second order aberrations of astigmatism and defocus in an aberration map to prescribe a correcting lens that eliminates second-order aberrations. Unfortunately, several studies show that eliminating the second-order Zernike aberrations may not optimize either the subjective impression of best-focus or the objective measurement of visual performance. This phenomenon may be explained by the lack of a universally-accepted metric of image quality to establish an objective optimum-focus state for an aberrated eye. A useful metric of optical quality for the human eye is one that is highly correlated with visual performance, the subjective judgment of best focus, or other tasks that are optically limited. Traditional metrics such as Strehl ratio, root mean squared (RMS) wavefront error are widely used in ophthalmic optics, but they are not well-suited as a metric that is related to the subjective judgment of best focus for the eye. In fact, recent data suggests that calculations based on RMS error are poor predictors of visual acuity for aberrated retinal images. Therefore, a system for prescribing and evaluating clinical optic prescriptions should employ metrics that are good objective indicators of subjective visual acuity for aberrated retinal images.
One method for generating an optimized optic prescription is described in U.S. Pat. No. 6,511,180 to Guirao & Williams (the “Guirao Patent”). The Guirao Patent describes an iterative method for finding the optimum sphere, cylinder and axis parameters by simultaneously optimizing a metric of image quality. However, this method requires a great deal of computing power to perform, and may not optimize the visual quality, due to the metric employed. Further, this method does not incorporate the many clinical considerations into a method for determining an optic prescription that best suits a patient, and this patent is confined to the use of a small subset optical quality metrics to determine a refraction. Therefore, a method allowing multiple uses of multiple metrics of vision quality would be appreciated, greatly increasing the flexibility and utility of automated refraction and assessment of the visual consequences of different refractive treatments.
Further, an eye-care practitioner must take into consideration patient data, environmental data, and apply clinical judgment to adjust the optically optimized prescription. Failure to include these inputs can cause numerous unacceptable prescriptions. Each of these steps often requires subjective decisions based upon clinical experience, and must be made after or in addition to the calculations for optical optimizations based upon aberration measurements. Therefore, a method or system incorporating these clinical and patient considerations is desired. Further, an automated method or system to perform this task in a more accurate fashion utilizing less computing power is desired.